The present invention relates to improved electromagnetic coupling devices. More specifically, the invention relates to an electromagnetic coupling device suitable for driving, for instance, sewing machines, which must be stopped quite frequently and must respond quickly in a variable speed operation.
Electric motors having frictional clutches and brakes which are designed to be stopped frequently, namely so-called "clutch motors," have been extensively employed to drive sewing machines especially industrial sewing machines. A clutch motor with an automatic needle positioner is known in the art in which the clutch motor is so designed that the needle can be stopped at a predetermined position by providing the clutch motor a positioning low speed drive function. This improved clutch motor has come into wide use in association with the provision of a automatic thread cutting device for sewing machines which together contribute greatly to improving the efficiency of sewing work.
However, such a clutch motor with an automatic needle positioner is disadvantageous in the following point. A variable speed characteristic is provided for the clutch motor by controlling the frictional clutch and the brakes, that is, by varying the slip condition of the clutch. It is difficult to set the optimum values of positioning speed and thread cutting speed when the clutch motor is operated in a low speed range and the frictional material of the clutch plates tends to wear out quickly which makes it necessary to inspect the clutch at frequent intervals.
In order to eliminate the above-described difficulties and to provide both an improved variable speed characteristic and an improved low speed characteristic, a technique has been proposed in the art in which a sewing machine is driven by a contactless variable speed motor, that is, an electric motor utilizing electromagnetic coupling. An example of such a motor is described in Japanese Pat. No. 699,108.
In order to frequently run and stop the electric motor driving the sewing machine, it is desirable that the inertia at the output shaft driving the sewing machine be lower than that at the rotary shaft of the electric motor preferably with the ratio of the two being less than 1/5. An electromagnetic coupling designed to meet such a requirement is disclosed in U.S. Pat. No. 3,910,211. An electromagnetic coupling for an electric motor in which the inertia at the output shaft is further decreased is disclosed in Japanese Utility Model No. 960,895.
However, electric motors which electromagnetic couplings of the type disclosed by these publications have not found practical use because of the following drawbacks. In the electromagnetic coupling of U.S. Pat. No. 3,910,211, a drive member is provided for the electromagnetic coupling by providing a drum and a cooling fan on the end face of a flywheel which is secured to the rotary shaft of the electric motor, a driven member or a follower of the electromagnetic coupling by fixedly securing to the output shaft a magnetic pole member having magnetic pole pieces confronting one another through a gap with the inner surface of the drum and a yoke supporting the magnetic pole piece. The magnetic pole member accordingly is U-shaped in partial section. An exciting coil secured to a part of the housing of the electric motor is held in a spaced relation with the U-shaped magnetic pole member of the follower. For an electric motor with an electromagnetic coupling of the type described in U.S. Pat. No. 3,910,211, the inertia at the output shaft is smaller than that at the rotary shaft when compared with an electric motor having an electromagnetic coupling in which the drum forming the electromagnetic coupling is secured directly to the output shaft and the inductor of the electromagnetic coupling is secured to the rotary shaft of the motor.
However, if the magnetic pole member is secured to the output shaft in the case where, as in the above-mentioned U.S. patent, the drum is arranged at the rotary shaft side, it is necessary to provide a predetermined relatively large size magnetic path sectional area for the magnetic pole member. Accordingly, the resulting construction is necessarily bulky and high in inertia because the magnetic pole member is able to effectively pass magnetic flux induced by the exciting coil without loss. Because of this, the inertia at the output shaft is not decreased as desired and the ratio of the output shaft torque to the output shaft inertia is undesirably small. Thus, an electric motor having an electromagnetic coupling of the type described in U.S. Pat. No. 3,910,211 is not practically applicable to a case where the motor should be accelerated or decelerated quickly such as on the order of 0.1 to 0.2 seconds as required for motors used for driving sewing machines.
Furthermore, in an electric motor having an electromagnetic coupling as in the above-mentioned U.S. patent, the flywheel serving as the drum must be provided separately from the magnetic pole member and as a result the number of components and the number of manufacturing steps required are increased. As the flywheel is arranged axially of the magnetic pole member, the longitudinal dimension of the electric motor must be increased accordingly.
In addition, in the electric motor having an electromagnetic coupling as described in this patent, the cooling fan is attached to or formed integrally with the flywheel with the flow of air provided by the cooling fan directed from the outer end of the output shaft toward the outer end of the rotary shaft of the electric motor. Since the operator must position himself at the unloaded side of a sewing machine when the machine is being driven by the electric motor, the flow of air is directed towards him. This can be an unpleasant annoyance or even a potential health hazard.
In the aforementioned Japenese Utility Model No. 960,895 there are proposed two types of electromagnetic couplings which are different from that in the above-mentioned U.S. patent. In one of the two types of electromagnetic couplings, the drive member of the electromagnetic coupling is constituted by a magnetic pole member which is U-shaped in partial section and which has magnetic pole pieces arranged to alternately engage with one another in a circumferential direction, a magnetic path forming member mounted on the magnetic pole member with a non-magnetic support confronting the magnetic pole pieces with a gap therebetween, and an exciting coil fixedly provided for the magnetic pole member. The drive member is fixedly secured to the rotary shaft of the electric motor at the center of the magnetic pole member. The follower for the electromagnetic coupling is constructed of an eddy current generating plate mounted on the output shaft in such a manner that the eddy current generating plate is interposed in the gap between the magnetic pole pieces and the magnetic path forming member.
An electromagnetic coupling of this type has a highly responsive speed variation characteristic because the electromagnetic coupling follower is constructed with a disc-shaped eddy current generating plate so that the inertia of the output shaft is low. However, it is still disadvantageous in that, as the exciting coil is integrally secured to the magnetic pole member, the exciting coil must be rotated as a part of the drive member with the rotary shaft. This involves a problem in that terminals such as slip rings must be provided for applying current to the exciting coil. The use of slip rings to supply current to the exciting coil makes the construction of the electric motor intricate and is accompanied by a necessity for inspecting the slip rings for wear. Thus, the employment of such an electromagnetic coupling has not proved practical.
In another type of electromagnetic coupling, the use of the slip rings is eliminated in order to overcome the above-described difficulties. Such an electromagnetic coupling is substantially similar to that described in the specification of Japanese Pat. No. 699,108. In this electromagnetic coupling, a yoke which is U-shaped in partial section is formed by separating a part of the magnetic pole member so as to accommodate the exciting coil and the yoke is mounted in the housing of the electric motor in such a manner as to confront the remaining part of the magnetic pole piece which is attached to the rotary shaft with a gap therebetween thus forming a magnetic circuit.
With this arrangement, no slip rings are needed because the exciting coil remains stationary. However, the arrangement has another drawback as follows. As described above, a part of the magnetic pole member secured to the rotary shaft of the electric motor is separated and used as the yoke where the exciting coil is provided. With this construction the weight of the magnetic pole member is reduced by the weight of the part thus separated so that the inertia of the rotary shaft is correspondingly decreased. Since the desired speed variable character is established by the electromagnetic coupling in a contactless clutch arrangement, even if the inertia at the output shaft is reduced, the reduction of the inertia of the rotary shaft cancels the reduction of the inertia of the output shaft as a result of which the performance is unavoidably lowered. Thus, such an electromagnetic coupling is not useful with an electric motor which must be frequently stopped and which requires a high responsive variation characteristic as in the case of an electric motor driving a sewing machine.
This difficulty may be eliminated by employing a flywheel. However, the use of flywheel produces additional problems in that the number of components and the number of manufacturing steps are increased by as many as are required for the addition of the flywheel and furthermore it is necessary to provide a space for installing the flywheel as a result of which the overall dimensions of the electric motor are increased.
In an electromagnetic coupling of this type, the magnetic pole member confronts the yoke in the axial direction and therefore the magnetic attraction force of the yoke is imparted to the magnetic pole member. Thus, it is necessary to provide some way of counterbalancing such a force.
In each of the two types of electromagnetic couplings disclosed in the above-described Japanese utility model, the output shaft follower is constituted by a disc-shaped eddy current generating plate. The eddy current generating plate is disadvantageous in that it has a low rigidity because of its configuration and is liable to be deformed by thermal stress due to thermal loss. Because of this difficulty, it is impossible to make the gap small between the magnetic pole pieces and the magnetic path forming member and accordingly the performance is lowered because of the loss of magnetic flux in the gap.
It is stated in the specification of the above-described Japanese utility model that the magnetic pole pieces and the eddy current generating plate may be arranged in the gap in a radial direction instead of in an axial direction. However, it is submitted that what is intended by the statement is vague because no actual construction or example thereof is given. It is assumed that the statement refers to the formation of a cup-shaped eddy current generating plate. If this is true the problems of deformation of the eddy current generating plate and the decrease in the performance due to a magnetic flux loss in the gap which is limited by the deformation of the eddy current generating plate can be eliminated. However, the remaining problems mentioned above remain. No way of solving these problems is described in that specification.